this fraction. The MW suggested for F8-200 by the standards used in the Sephadex G-200 column ranged from 13 to 17K. In a preliminary study performed in 6.5% SDS-PAGE with the active F2-50 of hemolymph and intestinal homogenate of the unconcentrated samples, only a 14-17K band was detected (data not shown). When 10-fold concentrated F2-50 of intestinal homogenate was analyzed for protein components in SDS-PAGE, the sample revealed the presence of 5 major proteins of apparent MW of 75, 59, 42, 23, and 16K, together with other minor components. The protein having a MW in the 16K band appeared to be present at the highest concentration (Fig. 2). The intestinal homogenate was prepared without protease inhibitors to avoid toxicity. Therefore, part of the major or minor proteins detected might represent degradation products. The relative contribution of each of these components to the process of differentiation is under study at present. As F250 contained components under 75K, a contamination of F2-50 samples with the inhibitory fraction was possible, but the lack of inhibitory action suggests either negligible contamination, or neutralization of its activity by some other component. This work was supported by funds of Secretaria de Estado de Ciencia y Tecnologia y Universidad de Buenos Aires, Argentina. A.M.K. and S.M.G.C. are members of the Research Career of the Consejo Nacional de Investigaciones Cientificas y Tecnicas from Argentina. roteins detected might represent